The present invention relates generally to the field of optics and in particular to an interferometric system that may detect dynamic phenomenon in turbid media as well as particle movement velocity and direction using differential phase measurements.
Optical low-coherence reflectometry has been used extensively in ranging of optical components and more recently in optical coherence tomography (OCT) for imaging turbid media such as tissues. Previous application of OCT to image turbid media have relied on envelope detection of the interference fringe intensity to construct pixelated tomographic images. Phase sensitive detection of the interference fringe intensity can dramatically improve the resolution of optical low-coherence tomography, especially in objects in which reflectivity contrast is poor, or sub-wavelength changes in optical path length are being measured.
In a single-channel-fiber Michelson interferometer, accurate phase sensitive detection of the interference fringe intensity is difficult because of the presence of phase noise. Environmental perturbations, including temperature fluctuations, air currents, vibration, mechanical movements and phase modulation contribute to the phase noise. Elimination of phase noise in an interferometer is usually difficult and expensive. Accurate phase detection is especially difficult in optical fiber interferometers used to image tissue structures where environmental perturbations result not only in phase fluctuations for optical signals of each light polarization incident on the sample but also random variation between phases in orthogonal polarization channels. Moreover, random phase variations in optical fiber interferometers between orthogonal polarization channels results in a variable light polarization state incident on the sample.
Conventional methods for accurate phase sensitive detection of the interference fringe intensity require careful monitoring of any optical path length variations in the interferometer. Unfortunately, application of these methods is limited principally to bulk interferometric systems in air and are not easily adapted to fiber based instruments. One can dramatically reduce phase noise by performing differential phase measurement, since the common-mode phase noise is canceled. A fiber optic interferometer is described that allows measurement of the differential phase between two de-correlated optical signals incident on and returned from a turbid sample.
Differential phase measurement between optical signals in two decorrelated interferometric channels can be applied to measure accurately and image many phenomenon in turbid media. For example, the Doppler effect arises when the optical phase of light backscattered from an object is modified due to particle movement along the light propagation direction. Conventional Doppler optical coherence tomography is a useful tool for noninvasive fluid-flow measurements in highly scattering media. Two advantages of Doppler optical coherence tomography are high spatial resolution (3-15 microns) and extraordinary volumetric flow sensitivity (40-108 pL/s).
With Doppler optical coherence tomography one can monitor flow velocities at a discrete location in a highly scattering medium. In conventional Doppler OCT, flow velocity is estimated from the Doppler-shifted interference signal formed between light backscattered from the flowing fluid in the sample path and light retroreflected from the reference path in a Michelson interferometer. The measured Doppler shift is dependent on the difference between the incident and the scattered lightwave vectors, which is referred to as the Doppler angle.
Inasmuch as the Doppler-shifted interference signal is a function of the Doppler angle, inaccurate values of the angle will limit the accuracy of flow-velocity estimation. In many applications accurate estimation of the Doppler angle is difficult if not impossible, particularly when the flow is buried in a highly scattering medium (e.g., for in vivo blood flow monitoring). A dual probe-beam setup is used in ultrasonic color flow mapping (two transducers) and laser Doppler anemometry (two lasers) for accurate estimation of flow velocity. The claimed subject matter of the present invention includes a novel dual-channel optical low-coherence reflectometer (OLCR) in which two probe beams are oriented at a precisely known relative orientation that allows for measurement of differential phase of the optical signals backscattered from a turbid sample.
Measurement of the differential phase may be used to detect dynamically subwavelenth particle movements (blood flow), molecular re-orientations (neural action potential propagation), acoustic wave mechanical displacements, thermal induced motions, or electro-chemical reactions. Although we recognize the interferometric system described may be utilized to detect in turbid media any of the indicated dynamic phenomenon, we focus our description on differential phase measurements to determine particle movement velocity and direction.
Differential phase measurement in turbid or highly scattering media has long been recognized as a difficult problem. Generally, and in one form of the present invention, the use of two probe beams oriented at a preset orientation allows the measurement of the Doppler frequency shift in a highly light scattering sample.
More specifically, one form of the present invention is a dual channel optical reflectometer composed of a birefringent path coupler and an optical source path that is optically connected to the path coupler. Broad bandwidth light entering the path coupler is depolarized by propagating light through a Lyot depolarizer or equivalent acting optical component. Light entering the path coupler consists of two de-correlated optical signals in orthogonal polarization channels. The path coupler must separate light into birefringent sample and reference paths while maintaining energy separation and decorrelation of optical signals in the fast and slow fiber polarization channels. After entering the path coupler, light is split into birefringent reference and sample paths.
The reference path is optically aligned with a first collimating lens, and the collimating lens is directed into a scanning delay line. There is also a birefringent optical sample path that is also optically connected to the path coupler. The sample path is optically aligned with a second collimating lens, a polarization channel separator/combiner and a lens, to focus and direct optical beams into the turbid sample. In the current embodiment, a Wollaston prism is utilzed as a static polarization channel separator/combiner. In other embodiments, a Rochon prism or similar acting optical device may be used as a dynamic or static polarization channel separator/combiner. In the current embodiment, the Wollaston prism is oriented so that optical signals in the two orthogonal and de-correlated polarization channels are separated to propagate in different directions.
The relative orientation of the light propagation vectors of light in the two orthogonal and decorrelated polarization channels is specified by the cut angle and material birefringence of the Wollaston prism. In the current embodiment, the second lens is positioned so that the cut-interface of the Wollaston prism and the point in the sample under investigation are at conjugate image planes. In this configuration, light in the two decorrelated polarization channels are brought to a common focus in the turbid sample at the point under investigation. Light backscattered from the turbid sample is collected by the second lens and orthogonal polarization channels are reunited by the polarization channel combiner. In the current embodiment, the Wollaston prism serves as both a static polarization channel separator and combiner.
A birefringent optical detection path is optically connected to the path coupler. A second polarization separator divides light in the two decorrelated polarization channels into different propagation directions and optically coupled to first and second photoreceivers that produce first and second output signals, respectively. The first and second output signals pass through a bandpass filter and amplifier. An analog-to-digital converter is connected to the amplifier, and a computer is connected to the analog-to-digital converter.
Another form of the invention is a dual channel reflectometer composed of a path coupler and a source path that is further composed of a first birefringent optical fiber having first and second ends. The first end of the first optical fiber is optically coupled to a light source and splits the light source into a first and second decorrelated polarization channels with independent phase components. The second end of the first optical fiber is connected to a depolarizer.
A second birefringent optical fiber having a first and a second end, has its first end connected to the depolarizer, and the second end of the second optical fiber is optically connected to the path coupler. The path coupler must separate light into birefringent sample and reference paths while maintaining energy separation and decorrelation of optical signals in the fast and slow fiber polarization channels.
There is also a reference path that is composed of a third birefringent optical fiber having first and second ends. The first end of the third optical fiber is optically connected to the path coupler. The second end of the third optical fiber is optically aligned with a first collimating lens that collimates light emitting from the second end of the third collimating fiber into a scanning delay line that includes a galvanometer that allows a variable phase modulation frequency.
There is also a sample path composed of a fourth birefringent optical fiber having a first and second end. The first end of the fourth optical fiber is optically connected to the path coupler. The second end of the fourth optical fiber is optically aligned with a second collimating lens that collimates light emitting from the second end of the fourth fiber and propagates through a first polarization separator (e.g., Wollaston prism) and a focusing lens, where the focusing lens is aligned so that the two decorrelated polarization channels with independent phase components are focused at a single point in a sample.
There is also a detection path composed of a fifth birefringent optical fiber having first and second ends. The first end of the fifth optical fiber is optically connected to the path coupler. The second end of the fifth optical fiber is aligned with a third collimating lens that collimates light emitting from the second end of the fifth optical fiber onto a second polarization separator (e.g., Wollaston prism) that separates the light from the fifth optical fiber into a first and a second beam, each beam corresponding to one phase component.
A first photodetector detects the first beam and produces a first output signal, and the second photodetector detects the second beam and produces a second output signal. The first and second output signals pass through a bandpass filter and amplifier. An analog-to-digital converter is attached to the filter-amplifer, and a computer is attached to the analog-to-digital converter.
In another form of the present invention, a birefringent optical fiber path is optically aligned with a collimating lens, a polarization channel separator/combiner (e.g., Wollaston prism) and a focusing lens such that the focusing lens focuses light beams corresponding to the two decorrelated polarization channels to a single point in the turbid sample.
In another form of the present invention, a birefringent optical fiber sample path is optically aligned with a collimating lens, a polarization channel separator/combiner (e.g., Wollaston prism) that produces two beams, and a focusing lens. The two beams are collinear and contain decorrelated optical signals that are longitudinally displaced relative to each other. xe2x80x9cLongitudinal displacementxe2x80x9d in this application means that the points of interest in the two beams are along the same line from, but at different distances from, a point of reference. The longitudinal displacement may be static or time-dependent and may be (but not necessarily) implemented using a static or time-dependent polarization separator/combiner in the sample path.
In another form of the present invention a birefringent optical fiber sample path is optically aligned with a collimating lens, and a polarization channel separator/combiner. Optical signals in the two separated beams are decorrelated. The polarization channel separator/combiner produces two beams, wherein the two beams are horizontally separated relative to one another, and the horizontal separation is time dependant. xe2x80x9cTime dependantxe2x80x9d in this application means that the degree of separation between the two beams changes as function of time. xe2x80x9cHorizontalxe2x80x9d is defined in this application as being the plane containing the two beam propagation directions. The horizontal displacement may be static or time-dependent and may be (but not necessarily) implemented using a static or time-dependent polarization separator/combiner in the sample path.
In yet another form the present invention provides a birefringent optical reference path that includes a phase modulator. The phase modulator may be, but is not limited to, a lithium niobate Y-waveguide electro-optic phase modulator.